Culture systems: sequential.

Methods for the culture of preimplantation human embryos evolved primarily from those used for mouse embryos. The initial method was usually culture in a single medium in microdrops of medium under oil for 2-3 days before transfer. Subsequently, extended culture over the whole preimplantation period was used. The debate at present is which system is best, a sequential series of media to accommodate changes in physiology and metabolism of the embryo from a 1-cell zygote to the differentiated blastocyst stage or a single-step culture regime using the same culture medium throughout the preimplantation period. Aspects of the advantages and disadvantages of these two culture systems will be discussed.